The head-neck motor system occupies a unique place in the motor control hierarchy. Depending upon behavioral context, it plays an important role in three quite distinct motor functions: head stabilization, saccadic and smooth tracking head movements and voluntary, non-visually guided head movements. We will focus on two strong descending projections to neck motoneurons: vestibulospinal and reticulospinal neurons. To begin filling major gaps in our knowledge about properties of these head control circuits, we propose experiments directed at the following three aims. 1. Characterize and compare patterns of neck muscle activity that monkeys use to execute vestibular stabilizing head movements, slow tracking head movements and rapid saccadic head movements in three dimensions. We will look for evidence that multiple muscle patterns are used in executing a movement. 2. Explore the role of single antidromically identified vestibulospinal and reticulospinal neurons in generating the head movements studied in Aim 1. Neurons will be classified according to the types of eye- and head-movement related signals they carry. We will examine how these neurons implement the brain's ability to cancel out vestibular head movement signals that arise from self-generated head movements. We will also look for evidence that multiple central controllers drive voluntary head movements. 3. Inject neuroanatomical tracers into the cervical spinal cord, brainstem reticular formation or regions of cerebral cortex projecting to vestibular nuclei and reticular formation to visualize pathways involved in neck motor control in primates. Impaired control of head movement is a debilitating symptom of many neurological disorders including stroke, cerebral palsy, Parkinson's disease and vestibular neuropathy. By increasing our understanding of how the CNS controls the head, this project should open the way to better treatment of these disorders.